Many critical electrically powered systems include one or more backup battery units that provide backup power to the system in the event of a loss of utility power. Backup battery units, however, can fail latently, and therefore may not be capable of providing adequate backup power to the system in the event of loss of utility power. While the capacity of a backup battery unit can be periodically checked by measuring the total amount of power discharged by the backup battery unit from a full charge level to an empty charge level, in the event of loss of utility power while the backup battery unit is at the empty charge level the backup battery unit would have no power to provide to the system.